1. Field of the Invention
The present invention relates to pellicles for protecting photomasks from particulate contamination and more particularly to pellicles suitable for use with deep ultraviolet radiation as is present when using excimer lasers.
2. Description of the Prior Art
Pellicles are used as protective covers to keep particulate matter outside of a focal plane of an optical apparatus so that a desirable image is not disturbed. Pellicles generally comprise thin, transparent membranes or films of polymer stretched over an aluminum frame that is mounted to form a hermetically sealed, dust-free enclosure over a photomask reticle. Pellicles are widely used in semiconductor manufacturing of integrated circuit, both to protect photomasks from particulate contamination and to extend the mask life. Their major function is to eliminate soft defects and improve die yield. The use of pellicles by semiconductor manufacturers is reviewed in detail by Ron Iscoff in "Pellicles 1985: An Update," Semiconductor International (April 1985). Projection printing systems also use pellicles, as is described by Shea, et al., in U.S. Pat. No. 4,131,363, issued Dec. 26, 1978. A broad class of pellicles and a method for forming these pellicles is also described by Winn, in U.S. Pat. Nos. 4,378,953 and 4,536,240, issued on Apr. 5, 1983 and Aug. 20, 1985, respectively. The following U.S. Pat. Nos. also describe pellicles: 4,973,142, issued Nov. 27, 1990, to Edward N. Squire, and assigned to du Pont; 4,948,851, issued Aug. 14, 1990, to Edward N. Squire, and assigned to du Pont; 5,008,156, issued Apr. 1, 1991, to Gilbert H. Hong, and assigned to Exion; and 4,657,805, issued Apr. 14, 1987, to Y. Fukumitsu, and assigned to Asahi Chemical.
Nitrocellulose has been widely used in pellicle manufacturing. But nitrocellulose is not suitable for 248 nanometers (or shorter wavelengths) lithography because nitrocellulose is highly absorbing at 248 nanometers and will rapidly degrade. The use of nitrocellulose has also declined because nitrocellulose is highly flammable and must be stored in a wetted condition. Nitrocellulose is also hygroscopic, which makes manufacturing under humid conditions difficult. Thus as a finished product, pellicles of nitrocellulose wrinkle when wetted with water, which makes cleaning or storing under humid conditions a problem. A critical problem with nitrocellulose pellicles is that the nitrocellulose material itself does not transmit ultraviolet (UV) light well enough for use in modern equipment that depend on the use of deep ultraviolet light. Irradiation with ultraviolet light can also cause a nitrocellulose pellicle membrane to become discolored, thus reducing its transparency. Below two hundred and sixty nanometers, even non-discolored nitrocellulose transmits less than seventy percent (70%) of incident light. This limitation in nitrocellulose, and also in MYLAR, when used in pellicles, is discussed by R. Hershel, in "Pellicle Protection of IC Masks," A Report by Hershel Consulting, Inc. (August 1981).
Advances in lithographic processes used in manufacturing integrated circuits depend on reducing the wavelength of the incident ultraviolet light used in conjunction with pellicles. The development of a broadband pellicle capable of transmitting ultraviolet light is described by I. E. Ward and D. L. Duly, in "Optical Microlithography III: Technology for the Next Decade," SPIE, Vol. 470, pp. 147-154 (H. L. Stover, Editor), 1984. Ward and Duly describe an antireflective layer that is coated on at least one side of a pellicle, in order to reduce any optical interference. Such optical interference is typically caused by internal reflections of light within a pellicle and is evidenced by an oscillating behavior in the transmission spectrum of a pellicle. Proposed solutions to this particular problem have included applying antireflective coatings and controlling the thickness of the membrane. Antireflective coatings do not adhere well to a pellicle's surface. The imperfect adhesion often then results in cracking and flaking of the antireflective coating, thus ruining the pellicle.
U.S. Pat. No. 4,657,805, issued Apr. 14, 1987, to Fukumitsu, et al., discloses the use of thin fluoropolymer films to serve as antireflective layers for a pellicle. Multiple layers of the fluoropolymer films are coated on a core layer pellicle to form a five-layer pellicle structure with the indexes of refraction of the various layers being chosen to reduce internal reflection and scattering.
The ultraviolet transmitting pellicles of Ward are described more completely in a series of three patents. U.S. Pat. No. 4,482,591, issued Nov. 13, 1984, discloses a pellicle comprised of polyvinyl butyral resin (PBR) and the use of a ring with an adhesive side to remove the pellicle from a wafer. U.S. Pat. No. 4,499,231, issued Feb. 15, 1985, discloses a pellicle comprising PBR and a dispersion of colloidal silica. U.S. Pat. No. 4,476,172, issued Oct. 9, 1984, discloses pellicles comprised of a PBR derivative that includes a silane moiety.
Problems also exist in the processes used to manufacture pellicles. For example, typically, a pellicle is formed by depositing a polymer solution on an inert substrate and then evaporating the solvent. This leaves the pellicle coated on the inert substrate. Removing the delicate pellicle from the substrate is a difficult, but a necessary step in the process. U.S. Pat. No. 4,536,240, issued to Winn, discloses a method for accomplishing this task by bonding a frame to the pellicle and then peeling the pellicle off the substrate. In conjunction with this procedure, a suitable release agent can be applied to the substrate prior to applying the fluoropolymer solution and thus aid in removing the pellicle. This procedure, however, results in a high number of pellicles being ripped during the removal step.
Duly, et al., in U.S. Pat. No. 4,523,974, issued Jun. 18, 1985, disclose a method for manufacturing a pellicle from polymethylmethacrylate (PMMA) that includes the steps of applying a gold film to the surface of an oxidized wafer, coating a thin layer of PMMA on the gold film, removing the PMMA and gold layers from the wafer and etching off the gold layer.
Microlithography trends for the last decade have been towards shorter and shorter wavelengths of ultraviolet radiation. The stepper radiation is changed from mercury G-line of 436 nanometers to I-line of 365 nanometers. A state-of-the-art stepper utilizes krypton fluoride emission at 248 nanometers and XE-F at 194 nanometers to delineate feature sizes around 0.3 micron.
Pellicles are well accepted by the photomask industry as an effective means of protecting the cleanliness of masks used in microlithographic processes. When masks are pelliclelized and used in transferring images of IC design on mask to wafer, the pellicles serve not only as a protective dust cover, but also as a part of the optics that do the lithographic imaging. Pellicle membranes must be photochemically stable to deep ultraviolet radiation, e.g., to wavelengths of 194 nanometers and 248 nanometers for excimer laser steppers. Pellicle membranes must be highly transparent into the deep ultraviolet range to guarantee high wafer throughput. Pellicle membranes must be very clean to ensure that no defects result in the wafers being processed. Pellicle membranes must be able to attach to an aluminum frame with appropriate adhesives and be strong, even at the typical thickness of 0.5 micrometers to 5.0 micrometers, to ensure the assembled pellicles are stout enough for ordinary use.
Although fluoropolymers have been described in the prior art as useful for deep ultraviolet pellicles with aluminum frames, a need nevertheless exists for a high yield method of casting the fluoropolymer films, a pellicle for eliminating bursting due to trapped air when the atmospheric pressure changes and a backliner that cooperates with robotics used in automated manufacturing facilities.
It is understood by those skilled in the art that pellicle manufacturing involves the coating of a glass substrate and a later peeling-off of a resulting membrane. Manufacturing yields are reduced by the tendency of the membrane to adhere too strongly to the glass substrate. In particular, this is a problem in the manufacture of deep ultraviolet (DUV) pellicles made of fluoropolymers.